JP2012108231A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that heats a fixing member by causing an electromagnetic induction heating layer provided on the fixing member to generate heat with a magnetic flux from an exciting coil, and heat-fixes an unfixed image on a sheet with the heated fixing member; and has improved accuracy for detecting short circuit of an exciting coil.SOLUTION: When high-frequency current is superimposed on current that flows through an exciting coil 104 due to discharge that occurs when some windings of the exciting coil 104 are in a rare short circuit condition, an image forming apparatus extracts the superimposed high-frequency current with a filter 86, amplifies an extracted signal with an amplifier 87, compares the voltage of the amplified signal with a threshold for determining the occurrence of high-frequency current due to rare short circuit with a comparator 88, and thereby determines the occurrence of rare short circuit from the comparison result.

Description

  The present invention relates to an image forming apparatus that heats an electromagnetic induction heat generating layer provided on a fixing member by a magnetic flux from an exciting coil and fixes an unfixed image on a conveyed sheet to the sheet by heat of the fixing member. .

  In the field of image forming apparatuses such as printers and copiers, those having an electromagnetic induction heating type fixing unit have been proposed. In the electromagnetic induction heating type fixing unit, an electromagnetic induction heating layer is provided on a fixing member such as a fixing roller or a fixing belt, and an excitation coil is arranged at a certain distance from the fixing member, and excitation is performed by supplying an alternating current. Generally, the electromagnetic induction heat generating layer is heated by a magnetic flux generated from a coil, and an unfixed image on a sheet such as a conveyed recording paper is fixed to the sheet by heat of a fixing member.

  An exciting coil is usually a multi-layered litz wire consisting of a large number of strands formed by coating a metal wire with a resin, for example, about 100 strands. Although used, the insulation coil of the wire may be damaged due to foreign matter entering the exciting coil, or the insulation withstand voltage may decrease due to the deterioration of the insulation film of the wire over time. May occur.

When a short circuit occurs between the windings, the electrical resistance value of the entire exciting coil decreases compared to when no short circuit occurs, and the amount of current flowing through the exciting coil increases as the electrical resistance value decreases. If the wire generates heat and the heated state continues for a long time, the temperature of the winding increases due to the heat generation, and the insulation film may be melted or smoked.
Therefore, in Patent Document 1, as a configuration for detecting a short circuit between windings of an exciting coil, the on-duty value of a switching element provided in a power supply unit for supplying an alternating current to the exciting coil is normal when the exciting coil is Due to the self-heating of the sampling coil, it gradually rises from the beginning to the end of the sampling period. A configuration for detecting a short circuit between windings of an exciting coil based on the magnitude of a change in on-duty value in the switching element is disclosed.

JP 2007-264085 A

However, the configuration of Patent Document 1 has a problem that the detection accuracy of a short circuit is poor when several of the winding strands are short-circuited. This is due to the following reason.
In other words, in the case of a short of several strands, only a part of the current flowing in the winding flows into the shorted portion, so a large amount of current is used as in the case where the strands are in contact with several tens of units and shorted. It is difficult to cause an overcurrent due to inflow. For this reason, the self-heat generation amount is reduced and the on-duty value is gradually increased when viewed from the entire exciting coil as in the normal state, and it is likely that a short circuit cannot be detected.

Even a short of several strands of wires does not change that the portion locally becomes hot due to current concentration in the shorted portion, and it cannot be said that there is no risk of smoke generation.
In addition, even if the high temperature portion is initially localized, the insulation film breaks down or the breakdown voltage decreases due to heat, eventually the short-circuited portion expands and the high temperature portion also expands. It can happen.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of further improving the detection accuracy of an excitation coil short circuit.

  In order to achieve the above object, an image forming apparatus according to the present invention heats an electromagnetic induction heat generating layer provided on a fixing member by a magnetic flux generated from an exciting coil, and converts an unfixed image on a conveyed sheet into the fixing member. An image forming apparatus for fixing to the sheet by the heat of the detector, wherein the detecting means detects an alternating current having a frequency corresponding to a high-frequency current associated with a discharge at the time of a short between windings in the exciting coil; Output means for outputting that a trouble has occurred in the exciting coil when a current is detected.

The detection unit may detect the current by supplying power to the excitation coil within a predetermined period other than during the fixing operation of the unfixed image on the sheet.
Further, during the fixing operation, a first magnitude of electric power is supplied to the exciting coil to maintain the temperature of the fixing member at a fixing temperature necessary for the fixing operation. The current may be detected by supplying electric power having a second magnitude larger than the magnitude to the exciting coil.

  Here, during a period other than during the fixing operation, the power supplied to the exciting coil is suppressed to a third power smaller than the first magnitude, and the temperature of the fixing member is made higher than the fixing temperature. The power saving mode for lowering the power saving temperature can be executed, and the predetermined period other than during the fixing operation is a period during execution of the power saving mode, and the detection means is during execution of the power saving mode, When detecting the current, the magnitude of power supplied to the exciting coil is switched from the third magnitude to the second magnitude for a predetermined time, and then returned to the third magnitude. Also good.

The detecting means may detect a current having a frequency corresponding to the high-frequency current by detecting a current flowing through the exciting coil.
Further, a rectifying circuit that rectifies an AC voltage from an external commercial power source and supplies the rectified current to the exciting coil and a resonance circuit that is connected in parallel to the exciting coil and forms a resonant circuit with the exciting coil. A power supply unit including a capacitor and a switch element connected in series to the exciting coil and opened and closed at a predetermined cycle higher than the frequency of the AC voltage, and the power supply unit is configured when the switch element is closed. A circuit configuration in which a current flows through a series circuit including the exciting coil and the switch element, and the current flowing through the switch element is interrupted by switching from a closed state to an open state, and a current flows through a resonance circuit including the exciting coil. The detecting means may detect the current flowing through the resonance circuit as the detection of the current flowing through the exciting coil.

  Here, the voltage waveform of the current flowing through the resonance circuit at the off time when the switch element is open rises from the start of the off time, turns to a drop after a peak, and reaches the end of the off time. The waveform has a waveform, α1 is the rising start time, α2 is the falling end time, γ1 is the time when a predetermined time β1 has elapsed from the time α1, and the time is a predetermined time β2 from the time α2. Where γ2 is a time point traced back, the detection means corresponds to the high-frequency current based on the detected voltage value of the waveform portion between the time point γ1 and the time point γ2 in the voltage waveform in the OFF time of the switch element. A current having a frequency to be detected, and based on the detected value of the voltage of the waveform portion between the time point α1 and the time point γ1 and between the time point γ2 and the time point α2. The detection of the current having the frequency corresponding to the high frequency current may not be performed.

Here, the predetermined times β1 and β2 may be times corresponding to 10% of the off time.
Further, the voltage waveform of the current flowing through the resonance circuit in the off time when the switch element is open rises from the start of the off time, changes to a drop through a peak, and reaches the end of the off time. The detection means has a frequency corresponding to the high-frequency current based on a detection value of a voltage of a waveform portion that is a voltage equal to or higher than the peak or a predetermined value in a voltage waveform in an off time of the switch element. Current detection may be performed.

  Also, a rectifying circuit that rectifies an AC voltage from an external commercial power source and supplies the rectified current to the exciting coil and a resonance circuit that is connected in parallel to the exciting coil and forms a resonant circuit with the exciting coil. A capacitor, a switching element connected in series to the exciting coil, and a series circuit of the exciting coil and the switching element, which is interposed between the exciting coil and the switching element, and has a higher withstand voltage that is higher than the switching element. A power supply unit having a switch element, wherein the power supply unit is switched from the open state to the closed state by switching the switch element from the open state and the high withstand voltage switch element to the closed state. Current flows through the high-breakdown-voltage switch element to a series circuit composed of the switch elements, and the high-breakdown-voltage is generated when a predetermined time has elapsed since the switching When the switch element is switched from closed to open, the current flowing through the high-breakdown-voltage switch element is interrupted, and the current flows through the resonance circuit including the excitation coil. The detection means includes the current flowing through the excitation coil. As a detection of this, a current flowing in the resonance circuit may be detected.

  Here, the voltage waveform of the current flowing through the resonance circuit after the time when the high-breakdown-voltage switch element is switched from closed to open becomes a mountain-shaped waveform that rises from the switching time and turns downward after a peak. In the voltage waveform, the detection means may detect a current having a frequency corresponding to the high-frequency current based on a detection value of a voltage of a waveform portion that is a voltage equal to or higher than the peak or a predetermined value.

  Furthermore, a demagnetizing coil that generates a magnetic flux for canceling the magnetic flux from the exciting coil is provided, the exciting coil is extended along a sheet width direction orthogonal to the sheet conveying direction, and the demagnetizing coil is This is the end of the exciting coil in the extending direction, and in the extending direction, the coil is disposed adjacent to the coil portion corresponding to the non-sheet passing area outside the sheet passing area of the sheet having a predetermined size or less. The detecting means may detect a current having a frequency corresponding to a high-frequency current accompanying the discharge by detecting a current flowing through the degaussing coil.

  Here, a rectification circuit that rectifies an AC voltage from an external commercial power source and supplies the rectified current to the excitation coil and a parallel connection to the excitation coil, and configures a resonance circuit between the excitation coil A power supply unit including a resonance capacitor and a switch element connected in series to the excitation coil and opened and closed at a predetermined cycle higher than the frequency of the AC voltage, and the power supply unit is configured when the switch element is closed. A circuit configuration in which a current flows through a series circuit composed of the excitation coil and the switch element, and the current flowing through the switch element is cut off by switching from a closed state to an open state, and a current flows through a resonance circuit including the excitation coil. The detecting means may detect a current flowing through the degaussing coil by electromagnetic induction when a current flows through the resonance circuit.

  In addition, the detection unit may detect the current only when the value indicating the cumulative value of the power feeding time to the exciting coil reaches a predetermined value within the predetermined period.

  In this way, even when the excitation coil short-circuit occurs at an extremely small local portion such as several windings at the initial stage, the frequency corresponding to the high-frequency current associated with the discharge due to the short-circuit is reduced. By detecting the alternating current, it becomes possible to detect the short circuit quickly and accurately, to notify the user that trouble has occurred due to the short circuit in the excitation coil, or to cut off the power supply to the excitation coil. It becomes possible to do.

1 is a diagram illustrating an overall configuration of a printer according to a first embodiment. It is a figure which shows the structure of the control part provided in a printer. It is a figure which shows the mode of the electric power supply during the printing execution in a normal mode. It is a figure which shows the mode of the electric power supply in power saving mode (at the time of a sleep). It is a figure which shows the outline of a circuit structure of the inverter power supply part provided in a printer. FIG. 6 is a diagram illustrating an example of a waveform of a collector current Ic flowing between a collector and an emitter of a switching element and a collector voltage Vc applied to the collector while the switching element of the inverter power supply is switched during printing in a normal mode. is there. It is a figure which shows the example of the waveform of the collector current Ic and the collector voltage Vc while the switch element is switched at the time of coil abnormality detection. It is a figure which shows the example of a mode that the external appearance of the whole excitation coil provided in the fixing part of a printer, and the excitation coil have generate | occur | produced. It is a figure which illustrates the voltage waveform etc. of the frequency component of the high frequency current taken out with the filter provided in the inverter power supply part in the area of 1 period T of the switching frequency in a switch element. It is a flowchart which shows the content of the process for judging the presence or absence of a rare short having generate | occur | produced in the exciting coil. It is a flowchart which shows the content of the subroutine of a coil abnormality detection process. It is a figure which shows the mode of the electric power supply at the time of a coil abnormality detection process. It is a figure which shows the magnitude | size relationship between the width | variety of the sheet | seat passed through an exciting coil and a degaussing coil while showing the structure of the exciting coil and degaussing coil which concern on Embodiment 2. FIG. 6 is a diagram illustrating a circuit configuration of an inverter power supply unit according to a second embodiment. FIG. 6 is a diagram showing a circuit configuration of an inverter power supply unit according to a third embodiment. It is a figure which shows the example of the waveform of the voltage Va applied to the collector current Ic and the high voltage | pressure-resistant relay while the switch element is switched at the time of coil abnormality detection in Embodiment 3, and the timing chart of a switch element and a high voltage | pressure-resistant relay.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described using a tandem color digital printer (hereinafter simply referred to as “printer”) as an example.
<Embodiment 1>
(1) Overall Configuration of Printer FIG. 1 is a diagram showing the overall configuration of the printer 1.

  As shown in the figure, the printer 1 forms an image by a known electrophotographic system, and includes an image forming unit 10, an intermediate transfer unit 20, a feeding unit 30, a fixing unit 40, and a control unit. 50, an operation panel 60, a low voltage power supply unit 70, and an inverter power supply unit 80, connected to a network (for example, LAN), and an instruction to execute a print (print) job from an external terminal device (not shown) Is received, color image formation of yellow (Y), magenta (M), cyan (C) and black (K) is executed based on the instruction.

  The image forming unit 10 includes image forming units 10Y to 10K corresponding to Y to K colors, respectively. The image forming unit 10Y includes a photosensitive drum 11Y and a charger 12Y, an exposure unit 13Y, a developing unit 14Y, a primary transfer roller 15Y, a cleaner for cleaning the photosensitive drum 11Y, and the like disposed around the photosensitive drum 11Y. A Y toner image is formed on the photosensitive drum 11Y through known charging, exposure, and development processes. This configuration is the same for the other image forming units 10M to 10K, and corresponding color toner images are formed on the photosensitive drums 11M to 11K.

The intermediate transfer unit 20 includes an intermediate transfer belt 21 that circulates in the direction of the arrow.
The feeding unit 30 feeds the recording sheets S from the paper feeding cassette to the transport path 35 one by one. For example, the sheet feeding cassette may be configured to store sheets S of a certain size, or to be configured to switch and store different size sheets S for each size.
The toner images formed on the photoconductive drums 11Y to 11K are subjected to the action of electrostatic force due to the electric field generated between the transfer rollers 15Y to 15K and the photoconductive drums 11Y to 11K at the transfer positions of the photoconductive drums 11Y to 11K. Thus, primary transfer is performed on the intermediate transfer belt 21 that circulates. At this time, the image forming operation for each color is executed at different timings so as to be multiple-transferred to the same position on the intermediate transfer belt 21.

  The sheet S is conveyed from the feeding unit 30 at the timing of the image forming operation, and the sheet S is interposed between the intermediate transfer belt 21 and the secondary transfer roller 22 that is in pressure contact therewith. The color toner images on the intermediate transfer belt 21 are collectively transferred onto the sheet S under the action of electrostatic force generated by the electric field generated by the secondary transfer voltage applied to the secondary transfer roller 22. Next transferred. The sheet S after the secondary transfer is sent to the fixing unit 40.

The fixing unit 40 is based on an electromagnetic induction heating method, and includes a fixing roller 101, a pressure roller 102, a fixing belt 103, an excitation coil 104, a fixing motor 105, and the like.
The fixing belt 103 includes an electromagnetic induction heating layer 108 (FIG. 3) having a substantially cylindrical shape, a release layer 109 (FIG. 3) and the like laminated on the outer peripheral surface thereof.
The fixing roller 101 is disposed inside the circulation path of the fixing belt 103.

  The pressure roller 102 is pressed against the fixing roller 101 from the outside of the circulation path of the fixing belt 103 by a biasing force such as a spring through the fixing belt 103, and secures a fixing nip between the surface of the fixing belt 103. . Here, the pressure roller 102 is driven to rotate by receiving a driving force from the fixing motor 105, so that the fixing roller 101 and the fixing belt 103 are driven to rotate. Although the configuration in which the pressure roller 102 is the driving side and the fixing roller 101 and the fixing belt 103 are the driven side has been described, the relationship between the driving side and the driven side may be reversed. Further, both the fixing roller 101 and the pressure roller 102 may be rotationally driven.

  The exciting coil 104 is located outside the circulation path of the fixing belt 103 and is located at a certain distance from the fixing belt 103, and is supplied with electric power from the inverter power supply unit 80 to generate an electromagnetic induction heating layer of the fixing belt 103. Magnetic flux for generating heat is generated. The electromagnetic induction heat generating layer 108 generates heat by the magnetic flux from the exciting coil 104, and the fixing belt 103 is heated to a fixing temperature necessary for fixing.

The exciting coil 104 is formed by winding a number of strands formed by coating a metal wire with a resin, in which a litz wire consisting of about 100 strands is wound in multiple layers, The fixing roller 101 is formed so as to extend along the axial direction of the fixing roller 101 (FIG. 8).
With the fixing roller 101, the pressure roller 102, and the fixing belt 103 rotating, the sheet S after the secondary transfer passes through the fixing nip secured by the pressure contact between the fixing belt 103 and the pressure roller 102. The toner images (unfixed images) on the sheet S are fixed by being heated and pressed by the heated fixing belt 103 when passing through the fixing nip while being transported along the path 35. . The sheet S after fixing is discharged out of the apparatus via the discharge roller pair 38 and is stored on the storage tray 39.

  The operation panel 60 is located on the front surface of the apparatus main body and is arranged at a position where the user can easily operate. The operation panel 60 is a key group that is input and operated by the user, for example, a key for accepting job selection, a selection of a recording sheet size, In addition to a key for receiving, etc., a touch panel type liquid crystal display unit 61 for displaying a screen including various messages and receiving a touch input by the user is provided.

The low-voltage power supply unit 70 converts an AC voltage from an external commercial power supply into a direct current according to an instruction from the control unit 50 and supplies it to the image forming unit 10, the intermediate transfer unit 20, the feeding unit 30, the fixing unit 40, and the like. To do.
The inverter power supply unit 80 converts an alternating voltage from a commercial power supply into an alternating current having a predetermined frequency for supplying the excitation coil 104 in accordance with an instruction from the control unit 50, and supplies the alternating current to the excitation coil 104 of the fixing unit 40. .

(2) Configuration of Control Unit 50 FIG. 2 is a diagram illustrating a configuration of the control unit 50.
As shown in the figure, the control unit 50 includes, as main components, a communication interface (I / F) unit 51, a CPU 52, a ROM 53, a RAM 54, a fixing execution time measuring unit 55, and a fixing accumulated time storage. A unit 56 and a timer 57 are provided so that each unit can exchange signals and data with each other.

The communication I / F unit 51 is an interface such as a LAN card or a LAN board for connecting to a network, here, a LAN, and receives print job data sent from the external terminal via the LAN.
The CPU 52 reads a necessary program from the ROM 53 and controls the image forming unit 10, the intermediate transfer unit 20, the feeding unit 30, the fixing unit 40 and the like based on the print job data received via the communication I / F unit 51. Thus, the image forming operation is executed smoothly.

In addition, when a predetermined event such as an input operation by the user on the operation panel 60, an image forming operation, a communication process, or the like does not occur, the CPU 52 executes the power saving process on the assumption that the condition for shifting to the power saving mode is satisfied. To do.
In this power saving process, the low voltage power supply unit 70 is instructed to cut off the power supply to each module such as the image forming unit 10, the intermediate transfer unit 20, the feeding unit 30, the fixing unit 40, and the inverter power supply unit 80 is instructed. Thus, the power supply to the exciting coil 104 is cut off, and the process automatically shifts to a power saving mode (sleep) that saves power consumption during standby.

  When a power-saving canceling factor, for example, when a power-saving canceling key (not shown) on the operation panel 60 is pressed or when print job data is received from an external terminal occurs during the power-saving mode, the normal mode is entered. The power saving is canceled assuming that the transition condition is satisfied. Here, the low voltage power supply unit 70 and the inverter power supply unit 80 are instructed to cut off the power supply to the modules such as the image forming unit 10, the intermediate transfer unit 20, the feeding unit 30, the fixing unit 40, and the excitation coil 104. Solve the problem and restart the power supply. The state in which power saving is released is called normal mode.

FIG. 3 is a diagram illustrating a state of power supply during printing in the normal mode, and FIG. 4 is a diagram illustrating a state of power supply in the power saving mode (during sleep).
At the time of printing, as shown in FIG. 3, power from an external commercial power source 99 is supplied to each module such as the image forming unit 10 to the operation panel 60 and the control unit 50 via the low voltage power source unit 70, and an inverter power source unit It is also supplied to the exciting coil 104 via 80. Each module includes a motor (not shown) for rotationally driving the photosensitive drum 11, a fixing motor 105 for rotationally driving the pressure roller 102, a sensor for detecting the presence / absence of a sheet, a sheet size, and the like. It is. This figure shows a state where a maximum of 800 [W] power is output from the low-voltage power supply unit 70 and a maximum of 700 [W] power is output from the inverter power supply unit 80.

  During sleep, power from an external commercial power supply 99 is supplied to the control unit 50 via the low-voltage power supply unit 70 (arrow 71), as shown in FIG. Is supplied to a module part (not shown) for receiving the input (arrow 72), not supplied to other modules such as the image forming unit 10 (blocked), and is supplied to the exciting coil 104 via the inverter power supply unit 80. Is not supplied. The figure shows that 100 [W] of power is output from the low-voltage power supply unit 70 to the control unit 50 and the operation panel 60, but no power is output from the inverter power supply unit 80.

As described above, the low-voltage power supply unit 70 has a function of switching the supply destination of the output power between the normal mode and the sleep mode. Since the supply destination is only the control unit 50 and the operation panel 60 during the sleep mode, Also, the amount of power is switched to output a significantly smaller amount of power.
The inverter power supply unit 80 switches between supply and stop of power to the excitation coil 104 according to an instruction from the CPU 52. Since power supply is stopped during sleep, the fixing belt 103 does not generate heat, and the temperature of the fixing belt 103 is not maintained at the fixing temperature. If the sleep state continues for a long time, the temperature of the installation environment of the printer 1 ( It will drop to near room temperature.

Returning to FIG. 2, the ROM 53 stores a control program related to an image forming operation for executing a print job, a control program related to a coil abnormality detection process (described later) for detecting a short circuit of the exciting coil 104, and the like. The RAM 54 is used as a work area for the CPU 52.
The fixing execution time measuring unit 55 measures the driving time j using the timer 57 every time the fixing motor 105 is rotationally driven by a fixing operation, here, an image forming operation.

  The fixed fixing time storage unit 56 stores a cumulative value (fixed cumulative time) h of the driving time j measured by the fixing execution time measuring unit 55. Here, every time the fixing execution time measuring unit 55 measures the driving time j of the fixing motor 105, the measured time j is added to the cumulative fixing time h currently stored in the cumulative fixing time storage unit 56, and the addition is performed. The time (j + h) obtained in this way is rewritten (updated) as a new fixing cumulative time h. The fixing accumulation time h is used to determine whether or not to execute the coil abnormality detection process.

(3) Circuit Configuration of Inverter Power Supply Unit 80 FIG. 5 is a diagram showing an outline of the circuit configuration of the inverter power supply unit 80.
As shown in the figure, the inverter power supply unit 80 includes, as main components, a rectifier circuit 81, a resonant capacitor 82, a switch element 83, a switching CPU 84, and a coil short detection unit 85, and an input terminal 91, An external commercial power source 99 is connected to 92, and one terminal 118 and the other terminal 119 of the exciting coil 104 are connected to output terminals 93 and 94.

  The rectifier circuit 81 inputs an AC voltage from the commercial power source 99 via the input terminals 91 and 92, and rectifies and outputs the input AC voltage. One output terminal 95 of the rectifier circuit 81 is connected to the output terminal 93, and the full-wave rectified voltage is output from the output terminal 93 to the other terminal of the rectifier circuit 81 via the excitation coil 104, the output terminal 94, and the switch element 83. Applied to the series circuit connected to the output terminal 96 (ground).

The resonance capacitor 82 is connected in parallel to the excitation coil 104 and forms a resonance circuit with the excitation coil 104.
The switch element 83 is formed of an NPN type transistor, the collector of the transistor is connected to one output terminal 95 of the rectifier circuit 81 via the exciting coil 104, and the emitter is connected to the other output terminal 96 of the rectifier circuit 81. The base is connected to the output terminal 841 of the switching CPU 84.

  The switching CPU 84 has an input terminal 840 for inputting the instruction signal 112 from the CPU 52 of the control unit 50, an output terminal 841 for outputting a signal for instructing a switching operation to the base of the switch element 83, and a coil short circuit. An input terminal 842 for inputting a signal from the detection unit 85 is provided, and the switch element 83 is maintained in an on / off switching or off state at a predetermined cycle based on the input instruction signal 112.

In the present embodiment, the instruction signal 112 from the control unit 50 includes a signal A for instructing a switching operation in the normal mode, a signal B for instructing an off state during sleep, and a coil abnormality detection process. There are three types of signal C for instructing the switching operation.
<When signal A in normal mode is input>
When the signal A is input as the instruction signal 112, the switching CPU 84 outputs a pulsed drive signal (a signal that alternately repeats the H level and the L level (0 V)) from the output terminal 841. The frequency f of the signal A is set to 40 [kHz], for example.

When the drive signal output from the output terminal 841 of the switching CPU 84 is input to the base, the switch element 83 performs on / off switching based on the drive signal.
FIG. 6 shows waveforms of the current (collector current) Ic flowing between the collector and the emitter of the switch element 83 and the voltage applied to the collector (collector voltage) Vc while the switch element 83 is being switched during printing in the normal mode. It is a figure which shows the example of. The figure shows an example of current and voltage waveforms when switching is performed in the vicinity of the maximum voltage every half cycle in the voltage waveform after full-wave rectification. The same applies to FIG. 7 described later.

  As shown in FIG. 6, the collector current Ic has a current waveform that is turned on for a time t1 (for example, 25 [μsec]) in the switching period t (corresponding to one period of the frequency f of the signal A). The time t1 in the figure corresponds to the time when the switch element 83 is on and the time t2 is off in the period t. Here, the length of the time t2 is determined in advance so that the collector voltage Vc becomes a voltage having a peak value due to resonance in the time t2 in one cycle t. As the length of the time t2, for example, a time corresponding to about a half cycle of the resonance frequency is set.

When the switch element 83 is turned on (closed), the collector current Ic flows through the series circuit of the exciting coil 104 and the switch element 83. When the switch element 83 is turned off (open), the collector current Ic is cut off. The operation in which a current due to resonance flows through a resonance circuit including the excitation coil 104 and the resonance capacitor 82 is repeated by on / off switching.
The voltage waveform of the collector voltage Vc rises from the beginning of the off-time at the on-time t1 of the switch element 83, and rises from the start of the off-time at the off time t2 due to resonance action, and passes through a peak. It is a voltage waveform that generates a mountain-shaped waveform G that starts to descend and reaches the end of the off time. The peak-to-peak value Vpp of the collector voltage Vc is, for example, 600 [V].

By this on / off switching of the switch element 83, a resonance current (alternating current) flows through the excitation coil 104, and a magnetic flux for generating heat from the electromagnetic induction heat generating layer 108 of the fixing belt 103 is generated from the excitation coil 104. The heat generating layer 108 generates heat.
The CPU 52 of the control unit 50 constantly detects the surface temperature of the fixing belt 103 with a sensor (not shown). When the surface temperature of the fixing belt 103 falls below the fixing temperature, the switching CPU 84 outputs a signal A, When the fixing temperature is exceeded, the output of signal A is stopped.

  The switch element 83 performs on / off switching while the signal A is output, and does not perform on / off switching while the output of the signal A is stopped. Accordingly, the magnetic flux is generated from the exciting coil 104 during the output of the signal A and the electromagnetic induction heating layer 108 generates heat, and the generation of the magnetic flux from the exciting coil 104 is stopped while the output of the signal A is stopped. The electromagnetic induction heat generating layer 108 does not generate heat, and the heat generation of the electromagnetic induction heat generating layer 108 and its stop are repeated, whereby the surface temperature of the fixing belt 103 is maintained at the fixing temperature. Control for maintaining the surface temperature of the fixing belt 103 at the fixing temperature by repeatedly outputting and stopping the signal A is referred to as fixing temperature control.

<When signal B during sleep is input>
When the signal B is input, the switching CPU 84 continues to output an L level signal. As a result, the switching of the switch element 83 is stopped, the alternating current does not flow through the exciting coil 104, and the electromagnetic induction heat generating layer 108 of the fixing belt 103 does not generate heat.
<When the signal C at the time of coil abnormality detection is input>
When the signal C is input, the switching CPU 84 outputs a pulse signal in which the ON time of the switch element 83 is longer (the ON / OFF duty ON ratio is larger in one cycle) than when the signal A is input (in the normal mode). Output from the output terminal 841. As a result, the amount of current flowing through the excitation coil 104 in one cycle is greater than that in the normal mode, so that the peak value of the collector voltage due to resonance increases, that is, the voltage applied to the excitation coil 104 increases.

FIG. 7 is a diagram illustrating an example of waveforms of the collector current Ic and the collector voltage Vc while the switch element 83 is being switched when the coil abnormality is detected.
As shown in the figure, the collector current Ic is turned on for a time t11 (for example, 35 [μsec]) longer than the time t1 in the normal mode in one cycle T, and the time t21 is in the normal mode (t2). The peak-to-peak value (Vpp) of the collector voltage Vc is higher than Vpp in the normal mode, for example, 1000 [V]. In the example of the figure, the discharge waveform Z is superimposed on the portion showing the peak value of the waveform of the collector voltage Vc. This discharge waveform Z shows what is generated by a short circuit between the windings of the exciting coil 104.

FIG. 8 is a diagram showing an overall appearance of the excitation coil 104 and an example of a state in which a short circuit has occurred in the excitation coil 104.
In the figure, the adjacent portions A and B of the winding (litz wire) 115 of the exciting coil 104 are scratched 121 in the insulation film of the strands due to foreign matter mixing, and several pieces in the portion A are passed through the scratch 121. An example is shown in which a short circuit occurs due to slight contact between the metal wires of the strands and the metal wires of several strands in the portion B.

  In addition, when the insulating film becomes thin due to the fray 122 or the like generated in the portions C and D in the winding 115 and the insulating property of the portion is lowered, and a leak current flows between the portions C and D, a short circuit occurs. This example is also shown. This leakage current can also be caused by scratches on the insulation film of the wire. Usually, one of the shorts occurs. Hereinafter, the above short is referred to as a rare short.

  When a rare short occurs, a discharge often occurs between the portions A and B (between the portions C and D) at the moment when the short occurs, and when the discharge occurs, noise accompanying the discharge is generated in a specific frequency region, for example, As a high-frequency alternating current (high-frequency current) of 300 [kHz] to 300 [MHz], it is input to the winding 115 of the exciting coil 104 through the shorted portion and superimposed on the current flowing through the exciting coil 104. The voltage waveform of the superimposed high-frequency current corresponds to the discharge waveform Z in FIG.

  FIG. 7 shows an example in which the discharge waveform Z appears only in the middle waveform portion of the three waveform portions showing peaks in the waveform of the collector voltage Vc, but the other waveform portions are not shown. Actually, the discharge waveform Z appears in the same manner. The discharge between the short-circuited windings (element wires) does not occur unless the potential difference between the elements becomes larger than the air breakdown. In the figure, an example is shown in which the time when the potential difference at which discharge occurs is the peak of the collector voltage Vc (voltage applied to the exciting coil 104).

  In the figure, time β10 is the rise (rise) of the mountain-shaped waveform G at the off time (opening time) t21 when the switch element 83 is off in the voltage waveform of the collector voltage Vc every period T. When the time β1 and the time β3 corresponding to 10% of the time t21 are taken before and after the first time point α1, respectively, this corresponds to a time width obtained by adding the times β1 and β3 before and after the time.

In addition, time β20 is obtained when time β2 and time β4 corresponding to 10% of time t21 are taken before and after the time α2 at the end of the falling (falling) of the peak-shaped waveform G for each period T. Is equivalent to a time width obtained by adding the time β2 and β4 before and after that.
These times β10 and β20 correspond to times outside the detection time range in which the discharge waveform Z, that is, a high-frequency current due to a rare short, is detected within one cycle T. This is because, at times β10 and β20, the voltage value of the collector voltage Vc is significantly smaller than that at the peak, so that the high-frequency current due to the discharge hardly overlaps, and when another noise component is superimposed, This is because if β10 and β20 are included in the detection time, there is a risk of misdetecting this other noise component as a high-frequency current due to discharge.

Since the lengths of the times β10 and β20 and the respective start timings and end timings can be obtained in advance from the ratio of the cycle T to the times t11 and t21, the detection results of the collector voltage Vc at the times β10 and β20 for each cycle T. As described later, the determination of the presence or absence of high-frequency current due to a rare short circuit is not performed.
The times β1 and β2 are 10% of the time t21 as the predetermined times, respectively, but are not limited to this value. In addition, for example, a lower limit value of a voltage value range suitable for detection (a voltage range that is assumed to avoid erroneous detection due to another noise component) is determined in advance, and the voltage waveform G having a peak It is also possible to not perform voltage detection in a waveform portion whose voltage value is lower than the lower limit value (in other words, perform voltage detection in a waveform portion equal to or higher than the lower limit value (predetermined value)).

Referring back to FIG. 5, the switching CPU 84 has a circuit configured to output the above-described predetermined driving signal from the output terminal 841 in response to each input of the signals A to C from the CPU 52 of the control unit 50. The
The coil short detection unit 85 includes a filter 86, an amplifier 87, and a comparator 88.
The filter 86 is a band-pass filter that passes (takes out) a frequency component corresponding to the high-frequency current superimposed on the collector voltage Vc, and blocks (or attenuates) lower and higher frequency components. A signal indicating the frequency component of the current is output to the amplifier 87.

FIG. 9A is a diagram illustrating a voltage waveform 191 of the frequency component of the high-frequency current extracted by the filter 86 in the section of one cycle T of the switching frequency. This voltage waveform 191 corresponds to the discharge waveform Z. Signal component.
The frequency region of the high-frequency current is obtained in advance by experiments or the like (in the above example, 300 kHz to 300 MHz), and a bandpass that cuts components outside this region, for example, a switching frequency (in the above example, about 40 kHz) A filter is used. Note that it is only necessary to extract the frequency component of the high-frequency current. For example, a high-pass filter that passes the high-frequency current but blocks the signal component of the switching frequency can be used.

The amplifier 87 amplifies the voltage of the signal indicating the frequency component of the high-frequency current from the filter 86 and outputs the amplified voltage to the comparator 88.
The comparator 88 compares the voltage value of the signal from the amplifier 87 with a predetermined threshold value Q, and outputs an H level signal if the voltage value of the signal is equal to or higher than the threshold value Q. If smaller than that, an L level signal is output.

  FIG. 9B is a diagram illustrating a state in which the voltage value of the signal amplified by the amplifier 87 exceeds the threshold value Q. FIG. 9C is a comparison of the signal illustrated in FIG. 9B. FIG. 9 is a diagram showing a signal waveform 192 output when input to the device 88. As described above, if the amplified voltage of the high frequency current superimposed on the current flowing in the exciting coil 104 due to the discharge generated due to the rare short is equal to or higher than the threshold value Q, the H level signal is output from the comparator 88. Will be output.

  Here, the threshold value Q is a value that can detect a high-frequency current due to a rare short and can eliminate a noise component that is not related to the rare short, and the magnitude of the amplification factor of the amplifier 87. It is calculated | required beforehand by experiment etc. in consideration of the relationship. In this sense, it can be said that the filter 86, the amplifier 87, the comparator 88, and the like function as a detection unit that detects an alternating current having a frequency corresponding to a high-frequency current that accompanies a discharge during a short circuit between windings in the exciting coil 104. .

The output signal of the comparator 88 is output to the input terminal 842 of the switching CPU 84.
When the switching CPU 84 receives the output signal from the comparator 88, the switching CPU 84 monitors the received signal waveform 192 for each switching period T. Then, in the signal waveform G (FIG. 7) of the switch element 83 at the off-time t21, the time point that is back from the time point α2 by the time β2 from the time point γ1 (corresponding to the end point of β1) from the time point α1. If a high-level signal is detected more than a predetermined number for each switching period T for the waveform portion up to γ2 (corresponding to the start time of β2), it is determined that a rare short has occurred. If not, it is determined that a rare short has not occurred.

As a result, the detection result based on the voltage value of the collector voltage Vc at the time β10 and β20 is not reflected, and erroneous detection of the occurrence of rare short can be suppressed.
Returning to FIG. 5, the switching CPU 84 outputs the determination result, that is, that a rare short has occurred or has not occurred, from the output terminal 843 to the CPU 52 of the control unit 50 as the detection result signal 113. The CPU 52 can determine whether or not a rare short has occurred based on the detection result signal 113 from the switching CPU 84.

(4) Contents of Processing for Determining Whether Rare Short has Occurred FIG. 10 is a flowchart showing the contents of processing for determining whether or not a rare short has occurred, which is not shown by the CPU 52 of the control unit 50. It is repeatedly executed by being called by the main routine every predetermined time, for example, every several milliseconds.
As shown in the figure, it is determined whether or not the normal mode is set (step S1). If it is determined that the current mode is the normal mode (“YES” in step S1), it is determined whether or not the power saving mode transition condition is satisfied (step S2). The condition for shifting to the power saving mode is that the state where the predetermined event does not occur continues for a certain period of time.

If it is determined that the condition for shifting to the power saving mode is not satisfied ("NO" in step S2), the process returns. In this case, the normal mode is continued.
If it is determined that the condition for shifting to the power saving mode is satisfied (“YES” in step S2), the mode is shifted from the normal mode to the power saving mode (step S3), and the process proceeds to step S4. Along with the transition to the power saving mode, the signal B is output as the instruction signal 112 for instructing the inverter power supply unit 80 to perform switching. The inverter power supply unit 80 stops switching of the switch element 83 when the signal B is received. As a result, an alternating current does not flow through the exciting coil 104, and heating of the fixing belt 103 is stopped.

If it is determined in step S1 that the mode is not the normal mode, that is, the power saving mode (“NO” in step S1), the process proceeds to step S4.
In step S4, it is determined whether or not the flag F is zero. This flag F is set to 0 when the power saving mode is shifted to the normal mode in step S12 described later.
If it is determined that the flag F = 0 (“YES” in step S4), information indicating the cumulative fixing time h currently stored in the cumulative fixing time storage unit 56 is read (step S5).

  It is determined whether the read fixing accumulated time h is equal to or longer than a predetermined value h0 (step S6). Here, the predetermined value h0 is obtained in advance as a threshold value for determining whether or not the execution of the coil abnormality detection process is necessary based on the length of the fixing cumulative time h. For example, it may be a time corresponding to the driving time of the fixing motor 105 to be driven to perform printing on a predetermined number (such as 1000) of sheets S. Rare shorts often occur, for example, when the insulation resistance of the insulation film of the wire gradually decreases over a long period of time, and it is suitable to execute the coil abnormality detection process at certain intervals. It is.

If it is determined that the cumulative fixing time h ≧ the predetermined value h0 (“YES” in step S6), the cumulative fixing time h stored in the cumulative fixing time storage unit 56 is reset to 0 (zero) (step S7). ), A coil abnormality detection process is executed (step S8).
FIG. 11 is a flowchart showing the contents of a subroutine of coil abnormality detection processing.
As shown in the figure, the inverter power supply unit 80 is instructed to perform switching when an abnormality is detected (step S81). Here, the instruction signal 112 for instructing switching is switched from the signal B during sleep to the signal C for instructing switching during abnormality detection. The output of the signal C is continued for a predetermined time tc (for example, 1 ≦ tc ≦ 10 seconds). When the predetermined time tc elapses, the output is switched to the output of the signal B (returned to the original). During the predetermined time tc, when only the power supplied to the exciting coil 104 is viewed, it can be said that the power saving mode is temporarily canceled.

The inverter power supply unit 80 performs switching of the switch element 83 (switching with a longer on-time than the normal mode) for a predetermined time tc by receiving the signal C as described above.
FIG. 12 is a diagram illustrating a state of power supply during the coil abnormality detection process.
As shown in the figure, during the coil abnormality detection process, the low-voltage power supply unit 70 is the same as in the sleep mode, but the inverter power supply unit 80 has a longer switching time of the switch element 83 than the normal time, and thus consumes less power. It can be seen that a maximum of 1400 [W] is supplied in the example of FIG.

  As a result, an alternating current flows through the exciting coil 104 and heating of the fixing belt 103 is started. If a rare short has occurred in the exciting coil 104 at this time, the high frequency current superimposed on the current flowing in the exciting coil 104 is detected by the switching CPU 84 of the inverter power supply unit 80, the coil short detecting unit 85, etc., and the detection result Is output from the inverter power supply unit 80 to the CPU 52 of the control unit 50.

  Returning to FIG. 11, the CPU 52 receives the detection result signal 113 from the inverter power supply unit 80 (step S <b> 82), and determines whether or not a rare short has occurred from the received detection result signal 113 (step S <b> 83). Here, as described above, when the detection determination is made based on the determination result of the rare short circuit occurrence by the switching CPU 84 of the inverter power supply unit 80, it is determined that the rare short circuit has occurred, and the undetected determination is made. If so, it is determined that a rare short has not occurred.

If it is determined that a rare short-circuit has occurred (“YES” in step S84), a message indicating that an abnormality (trouble) has occurred in the exciting coil 104 is output. Here, a message indicating the occurrence of the abnormality is displayed on the operation panel 60. The information is displayed on the liquid crystal display unit 61 (step S85), and the process returns. In this case, the power is supplied to the liquid crystal display unit 61 even in the power saving mode.
The user can know that an abnormality (trouble) has occurred in the exciting coil 104 by viewing the message display on the liquid crystal display unit 61, and can notify a service person for inspection or repair. .

If it is determined that a rare short has not occurred ("NO" in step S84), the process returns without executing step S95 (skipping).
Returning to FIG. 10, when the coil abnormality detection process in step S8 is completed, the flag F is set to 1 (step S9), and the process proceeds to step S10.
If it is determined in step S6 that the fixing cumulative time h ≧ predetermined value h0 is not satisfied, that is, h <predetermined value h0 (“NO” in step S6), it is determined that the execution time of the coil abnormality detection process has not been reached and the coil abnormality is detected. Without executing the detection process (skip steps S7 and S8), the process proceeds to step S9, the flag F is set to 1, and then the process proceeds to step S10.

In step S10, it is determined whether or not a condition for shifting from the power saving mode to the normal mode is satisfied. The condition for shifting to the normal mode is that the above power saving cancellation factor has occurred.
If it is determined that the condition for shifting to the normal mode is not satisfied ("NO" in step S10), the process returns. In this case, when the process is started by the next call, the power saving mode is determined in step S1 (“NO” in step S1), and F = 0 is determined in step S4 (“NO” in step S4). In step S10, it is determined that the conditions for shifting to the normal mode are not satisfied ("NO" in step S10), and the process returns. Thereby, the power saving mode is continued, and the above processing is repeated during the power saving mode.

If it is determined in step S10 that the conditions for shifting to the normal mode are satisfied ("YES" in step S10), the mode is shifted from the power saving mode to the normal mode (step S11), and the flag F is set to 0 ( Step S12) and return. With the transition to the normal mode, the above fixing temperature adjustment control is executed.
When the process is started by the next call, the normal mode is determined in step S1 (“YES” in step S1), and when it is determined in step S2 that the power saving transition condition is not satisfied (“NO” in step S2). ), Return. Thereby, the normal mode is continued.

As described above, in the present embodiment, since the configuration of detecting an alternating current having a frequency corresponding to a high-frequency current associated with a discharge due to a rare short is detected in the present embodiment, a state where discharge can occur, that is, excitation It is possible to detect a short circuit of several strands in the coil 104, a short circuit due to a decrease in the insulation resistance of the strands, etc. at an early stage with high accuracy.
In addition, when the coil abnormality is detected, the voltage applied to the exciting coil 104 is set higher than that in the normal mode. Therefore, if a rare short occurs in the exciting coil 104, discharge due to the rare short is likely to occur. The detection accuracy can be further improved than the detection.

  In the above description, in the switching cycle of the switch element 83, it is not determined whether or not a rare short-circuit has occurred based on the voltage value of the collector voltage Vc at the times β10 and β20 (determination is prohibited). Absent. Depending on the device configuration, there is a possibility that there is a certain probability that high-frequency noise due to rare shorts is also superimposed between the times β10 and β20. In such a case, the time β10 and β20 may be included in the time for determining whether or not a rare short has occurred.

  6 and 7 described above, an example in which the peak portion of the half-wave voltage waveform is switched in the AC frequency F (50 or 60 Hz) of the commercial power supply 99 has been described. Rather, the current flowing through the excitation coil 104 every period T from the beginning of the cycle (starting point of voltage rise) to the end through the peak (end point of voltage falling) every half wave. The resonance current is generated every period T by the switching, and the generated resonance current is supplied to the exciting coil 104.

  Accordingly, if a short circuit occurs, a high frequency current is superimposed on the collector voltage Vc even at times other than the peak, so that the superimposed high frequency current can be detected. Since the voltage value becomes the smallest near the time points α1 and α2 (at the time when the voltage starts to rise and when the voltage ends), the peak where the voltage is maximum or the time zone around the peak It can be said that it is easy to detect a high-frequency current.

In the above description, the high-frequency current generated by the discharge due to the rare short between the windings of the exciting coil 104 is detected, but the occurrence of the discharge is not limited to the rare short between the windings.
For example, discharge may occur between the terminals due to poor contact due to insufficient tightening when the connection terminals 118 and 119 at both ends of the exciting coil 104 and the output terminals 93 and 94 of the inverter power supply unit 80 are fastened with screws or the like. obtain. Also in this case, if the high-frequency noise generated by the discharge is superimposed on the alternating current flowing through the exciting coil 104 as a high-frequency current, this high-frequency noise can be detected by the coil short detection unit 85.

  Further, not only the poor contact at the time of fastening, but also a part of the covered electric wire 117 (FIG. 8) that connects the winding 115 of the exciting coil 104 and the connection terminals 118 and 119 at the time of fastening work, for example. Bites into the device frame (not shown), a part of the coating is cut, the insulation resistance of the part of the coating is reduced, and the insulation resistance is reduced between the covered electric wire 117 and the device frame (ground). When discharge occurs, it can be a detection target as well as contact failure.

<Embodiment 2>
In the first embodiment, the high-frequency current superimposed on the current flowing in the excitation coil 104 is extracted by the filter 86. However, in the second embodiment, not the excitation coil 104 but the elongated excitation coil 104 is used. A high frequency current superimposed on a current flowing in a degaussing coil arranged so as to overlap one end in the longitudinal direction is extracted by a filter 86, and this point is different from the first embodiment. Hereinafter, in order to avoid duplication of description, the description of the same contents as those of Embodiment 1 is omitted, and the same components are denoted by the same reference numerals.

FIG. 13 is a diagram schematically showing the configuration of the excitation coil 104 and the degaussing coil 250 and the size relationship between the excitation coil 104 and the width of the sheet S passed through the demagnetization coil 250.
In the drawing, members such as the fixing belt 103 are omitted, but the horizontal direction in the drawing corresponds to the axial direction (sheet width direction) of the fixing roller 101, and the length L0 of the fixing belt 103 in the sheet width direction is the maximum. The sheet width L of the exciting coil 104 extending along the sheet width direction is longer than the length L0 of the fixing belt 103, which is longer than the sheet width L of the size sheet S (large size paper). . As a result, even when large-size paper is passed, the fixing belt 103 can be raised to the fixing temperature over the entire sheet width direction of the large-size paper.

In the present embodiment, even if the sheet S to be conveyed is a large size paper or a sheet having a sheet width smaller than or equal to a predetermined size (small size paper), one edge S0 in the sheet width direction is in the sheet width direction. A so-called one-side reference conveyance method is adopted in which the sheet is conveyed along the sheet feeding direction (upward direction in the figure) on the conveyance path 35 so as to match the reference position R provided on one side.
Therefore, when small size paper is passed, on the fixing belt, the small size paper does not pass through the sheet passing area L1 through which the small size paper passes in the sheet width direction in the entire sheet width L0. A sheet passing area L2 can be formed. In this case, in the paper passing area L1, heat is lost to the transported small size paper, so that the fixing temperature does not rise. However, in the non-paper passing area L2, heat is not lost to the small size paper. When the temperature is higher than the fixing temperature, the temperature rises excessively, and there is a possibility that the members disposed around the fixing unit 40 may be thermally deteriorated.

Therefore, in order to prevent an excessive temperature rise in the non-sheet passing region L2 when a small size sheet is passed, the degaussing coil 250 corresponds to the non-sheet passing region L2 on the end side in the extending direction of the exciting coil 104. It is arranged at the position to do.
The degaussing coil 250 is disposed adjacent to the coil portion corresponding to the non-paper passing region L2 of the exciting coil 104 so as to be in close contact with the exciting coil 104, and the magnetic flux for canceling the magnetic flux emitted from the coil portion is not passed through the non-paper passing region L2. (Demagnetization).

Due to the degaussing action of the degaussing coil 250, no heat is generated in the portion of the electromagnetic induction heating layer 108 of the fixing belt 103 corresponding to the non-sheet passing region L2 in the sheet width direction, and the temperature rise in the non-sheet passing region L2 is increased. Is prevented.
The winding 251 of the degaussing coil 250 is made of a litz wire, and one terminal 252 and the other terminal 253 are connected to the inverter power supply unit 201.

FIG. 14 is a diagram illustrating a circuit configuration of the inverter power supply unit 201 according to the second embodiment.
As shown in the figure, the inverter power supply unit 201 has basically the same configuration as that of the first embodiment, except that a circuit 202 for short-circuiting and opening the degaussing coil 250 is provided, and the filter 86 is provided. The difference is that the input voltage is not the collector voltage Vc but the voltage applied to the circuit 202.

  The circuit 202 includes a switch 203, an electric path 221 connecting one terminal 231 of the switch 203 and a terminal 211 connected to the terminal 252 of the demagnetizing coil 250, the other terminal 232 of the switch 203, and the other terminal of the demagnetizing coil 250. The electric circuit 222 is connected to the terminal 212 connected to the H.253. Here, the electric circuit 221 is electrically connected to the filter 86 via another electric circuit 223, and the electric circuit 222 is grounded.

The switch 203 is opened and closed by an on / off signal from the CPU 52 of the control unit 50. The CPU 52 outputs an on (closed) signal when the sheet S to be used is a small size sheet.
As a result, in the case of small size paper, the switch 203 is closed, and one end and the other end of the degaussing coil 250 are short-circuited and grounded. In this case, when a magnetic flux is generated from the exciting coil 104, a magnetic flux in a direction to cancel the magnetic flux is generated in the demagnetizing coil 250, and the magnetic flux emitted from the winding portion corresponding to the non-sheet passing region L2 in the exciting coil 104 is suppressed. As a result, the heating of the fixing belt 103 is suppressed in the non-sheet-passing area L2, and an excessive temperature rise is prevented. At this time, since the electric paths 221 to 223 are the same as the grounded state, the input voltage to the filter 86 becomes 0V.

  On the other hand, when the sheet S to be used is a large size paper, the CPU 52 outputs an off (open) signal. When the switch 203 is opened by an off (open) signal, the degaussing coil 250 is not short-circuited, and the degaussing action that cancels the magnetic flux generated from the exciting coil 104 does not work. As a result, the fixing belt 103 is heated over the entire sheet width L, and the large-size paper is fixed well.

  When the switch 203 is in an open state, a series circuit is formed from the electric circuit 222 (ground) to the filter 86 through the demagnetizing coil 250, the electric circuit 221, and the electric circuit 223. While an alternating current flows through the exciting coil 104, an induced electromotive force is generated in the degaussing coil 250 by the magnetic flux generated from the exciting coil 104. Therefore, the voltage due to the generated induced electromotive force is filtered through the electric paths 221 and 223. 86.

When a rare short occurs in the exciting coil 104, a high frequency current is superimposed on the current flowing in the exciting coil 104. This high frequency current is transmitted from the exciting coil 104 to the demagnetizing coil 250 by electromagnetic induction, A current having a high-frequency noise waveform similar to the high-frequency current (this current is also referred to as “high-frequency current”) is superimposed on the flowing current.
The high-frequency current superimposed on the current flowing through the degaussing coil 250 is input from the degaussing coil 250 to the filter 86 via the electric circuit 221 and the electric circuit 223. Since the high-frequency current is superimposed on the voltage input to the filter 86 as in the first embodiment, the high-frequency current is detected by the filter 86, the amplifier 87, and the comparator 88, and the detection result is shown. The detection result signal 113 can be sent to the CPU 52.

  In the coil abnormality detection process (step S8), the CPU 52 turns off (opens) the switch 203 and makes the power supplied to the exciting coil 104 larger than that in the normal mode, thereby causing a rare short circuit. The detection result of the high-frequency current superimposed on the current flowing through the degaussing coil 250 can be obtained from the inverter power supply unit 201 by making it easy to cause the discharge, and it can be determined whether or not a rare short has occurred.

  In the above description, an example of the one-side reference conveyance method has been described. For example, the position of the sheet S in the width direction of the sheet S is aligned with the reference position with respect to the position of the center of the conveyance path 35 in the width direction. Thus, the present invention can also be applied to a so-called center-based conveyance system configuration in which the sheet S is conveyed. In the center-based transport method, when passing small-size paper, there are non-paper passing areas on both sides of the fixing belt 103 in the sheet transport direction. Can be arranged.

When this configuration is adopted, it is possible to provide a circuit for short-circuiting and opening each demagnetizing coil and to connect both circuits or one circuit to the filter 86.
<Embodiment 3>
In the first embodiment, the alternating current is supplied to the exciting coil 104 by switching the switching element 83 of the inverter power supply unit 80 on and off in both the normal mode and the coil abnormality detection. However, in the third embodiment, In the normal mode, the switch element 83 is switched on / off, and when a coil abnormality is detected, the high-voltage relay is switched on / off instead of the switch element 83. This is different from the first embodiment.

FIG. 15 is a diagram illustrating a circuit configuration of the inverter power supply unit 301 according to the third embodiment.
As shown in the figure, the inverter power supply unit 301 basically has the same configuration as that of the first embodiment, except that a high voltage relay 303 is interposed between the output terminal 94 and the collector of the switch element 83. The voltage Va at the output terminal 94 is input to the filter 86, and the switching CPU 302 is provided with an output terminal 311 for outputting a signal for on / off switching to the high withstand voltage relay 303. The high voltage relay 303 is different in that switching control is performed separately.

The reason why the high withstand voltage relay 303 is provided in addition to the switch element 83 made of a transistor is as follows. That is, as described above, the high-frequency current due to the short circuit is easily generated by increasing the power supplied to the exciting coil 104 and increasing the voltage value of the alternating current flowing through the exciting coil 104 than in the normal mode.
Increasing the voltage value of the alternating current flowing through the exciting coil 104 requires increasing the withstand voltage of the switch element 83, but a normal transistor does not have a large withstand voltage. In order to increase the withstand voltage, a method of arranging a plurality of transistors in series is conceivable. However, the resistance at the time of transistor conduction increases, causing a problem that the inverter power supply efficiency during normal operation is deteriorated. Therefore, when a transistor is used, there is a limit to increasing the voltage value.

Therefore, in the third embodiment, at the time of detecting a coil abnormality, switching is also performed using the high withstand voltage relay 303 having a higher withstand voltage than the switch element 83 made of a transistor.
FIG. 16 is a timing chart illustrating the collector current Ic and the waveform of the voltage Va applied to the terminal on the side connected to the exciting coil 104 of the high withstand voltage relay 303 and the ON / OFF timing of the switch element 83 and the high withstand voltage relay 303 when the coil abnormality is detected. It is a figure which shows the example of a chart.

  As shown in the figure, when the switch element 83 is turned off (opened) and the high breakdown voltage relay 303 is turned on (closed) and the switch element 83 is switched from off to on (closed) (time tx), the collector current is increased. Ic begins to flow and the current value increases. A switching instruction (relay switching signal) from ON to OFF is output from the output terminal 311 of the switching CPU 302 to the high-breakdown-voltage relay 303 at an elapsed time ty from time tx to time t3.

  The high withstand voltage relay 303 takes more time to switch on and off than the transistor, and therefore, the high withstand voltage relay 303 is actually switched off from the switching instruction (time point ty) at the time point tz of the time t4 required for switching. When the high-voltage relay 303 is switched off, the collector current Ic does not flow, the current flows through the resonance circuit composed of the exciting coil 104 and the resonance capacitor 82, and the voltage waveform of the voltage Va becomes a mountain-shaped waveform G having a peak. .

If a rare short occurs in the exciting coil 104, the discharge waveform Z resulting from this is superimposed on the voltage waveform G.
In this case, a current having a frequency corresponding to the high-frequency current (discharge waveform Z) is detected based on the detected voltage value in the waveform portion after the time γ1 after the time β1 has elapsed from the time α1 at which the voltage waveform G starts to rise. Alternatively, the high-frequency current may be detected based on the detected value of the voltage of the waveform portion in the peak or a time zone around the time including the peak.

  The time t12 that is the sum of the times t3 and t4 corresponds to the time required for the current flowing through the exciting coil 104 to flow until it reaches a substantially maximum value, and is longer than the time t11. The time t4 is This is the time (delay time) required for switching the high voltage relay 303 as described above. Times t12 and t4 can be obtained in advance by experiments or the like. The time t3 is obtained by subtracting t4 from the time t12, and the relay switching signal is obtained at the time ty when the time t3 has elapsed from the on time tx of the switch element 83. Is output, the high withstand voltage relay 303 can be turned off at the time point tz when the amount of current to the exciting coil 104 reaches substantially the maximum.

  In this way, the peak value of the voltage Va can be made larger than that of the first embodiment, for example, and the voltage of the discharge waveform Z superimposed on the voltage waveform is also increased. Can be detected. Since the high withstand voltage relay 303 is turned off while the voltage waveform of the voltage Va has a peak-like waveform G, the switch element 83 has a large voltage exceeding the withstand voltage. There is no such thing.

In the above description, the time t12 is set to be longer than the time t11. However, the time t12 is not limited to this. For example, the time t12 is set to be equal to or shorter than the time 11 or the time t11 is set to a time equivalent to the time t12. You can also
When the voltage Va on which the high-frequency current is superimposed is input to the filter 86, the high-frequency current is extracted by the filter 86 and the voltage of the extracted signal is amplified by the amplifier 87, as in the first embodiment. The comparator 88 compares the amplified signal with the threshold value. Switching CPU 302 determines the presence or absence of a rare short based on the comparison result, and outputs the determination result to CPU 52 of control unit 50 as detection result signal 113.

In the normal mode, when the instruction signal 112 (signal A) from the CPU 52 of the control unit 50 is input to the input terminal 840, the switching CPU 302 outputs an on (H level) signal from the output terminal 311. The high voltage relay 303 is maintained in the on (closed) state.
The signal output from the output terminal 841 to the switch element 83 is the same as in the first embodiment.

As a result, only when the signal A is input in the normal mode, an alternating current flows through the exciting coil 104 by the on / off switching of the switch element 83 and the fixing belt 103 is heated.
In the sleep mode, when the instruction signal 112 (signal B) from the CPU 52 of the control unit 50 is input to the input terminal 840, the switching CPU 302 outputs an off (L level) signal from the output terminal 311, and the high withstand voltage. The relay 303 is maintained in an off (open) state. As a result, the alternating current does not flow through the exciting coil 104, and the heating of the fixing belt 103 is stopped.

  Thus, by providing the high voltage relay 303 separately from the switch element 83, it becomes easy to detect a rare short. In the initial stage of short-circuiting, for example, when the pinhole of the insulating film is very small at first, but spreads due to the influence of heat over time, the alternating current flowing in the exciting coil 104 is first opened. Even if a short circuit cannot be detected because the voltage is low and a rare short cannot be detected at that time, a rare short can be detected if a high voltage is applied as in this embodiment. It becomes possible to detect rare shorts earlier.

The high withstand voltage relay 303 is not particularly limited as long as the withstand voltage is higher than the maximum value of the voltage generated in the exciting coil 104. Further, it is not necessary to be a relay, and a switch element having a higher withstand voltage than the switch element 83 can be used.
In the above, the switching element 83 and the high voltage relay 303 are sequentially switched on and off to output only one mountain-shaped voltage waveform G, and the high-frequency current (discharge waveform Z) superimposed on the voltage waveform G is output. However, the present invention is not limited to this.

  For example, by executing the above operation continuously and repeatedly, the voltage waveform G having the number of repetitions is output, and the high frequency current superimposed on each voltage waveform is detected from each voltage waveform, A configuration (corresponding to the configuration of the first embodiment) for determining whether or not a rare short has occurred can also be taken. Note that the configuration in which a rare short is detected by one of the voltage waveforms can be applied to the first embodiment, for example.

Furthermore, in the above, the inverter power supply unit has a resonance circuit. However, the present invention is not limited to this, and the inverter power supply unit can be applied to a power supply unit having a circuit capable of supplying an alternating current to the excitation coil 104. Moreover, although the example which uses a transistor as a switch element was demonstrated, it cannot be overemphasized that the element in general which can switch electricity supply and interruption | blocking by opening and closing can be used.
The present invention is not limited to an image forming apparatus, and may be a method of detecting a rare short and outputting the detection result. The method may be a program executed by a computer. The program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It can be recorded on various computer-readable recording media, and may be produced, transferred, etc. in the form of the recording medium, wired and wireless various networks including the Internet in the form of programs, In some cases, the data is transmitted and supplied via broadcasting, telecommunication lines, satellite communications, or the like.

<Modification>
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications may be considered.
(1) In the above embodiment, the execution condition of the coil abnormality detection processing is set to be within a predetermined period other than during the fixing operation and the power saving mode and the fixing cumulative time h is equal to or longer than the predetermined value h0. It will never be done.

  For example, if the cumulative fixing time h is regarded as one of the values indicating the cumulative value of the power feeding time to the exciting coil 104, a value included in the index value, for example, an image forming operation is performed instead of the cumulative fixing time h. One of a cumulative value (cumulative image forming operation time) of accumulated time (time from print instruction to the last sheet discharge), a cumulative number of sheets printed, a cumulative rotation time of the photosensitive drum, etc. may be used. good.

  Further, it is possible to adopt a configuration in which the coil abnormality detection process is executed every time the mode is shifted to the power saving mode, regardless of the fixing accumulation time h. In this case, the execution time may be any time during the power saving mode. Further, after the transition to the power saving mode (step S3 in FIG. 10), it may be after a predetermined time has elapsed. As the predetermined time, for example, it is assumed that the temperature of the fixing belt 103 is required to decrease from the fixing temperature to the room temperature (apparatus installation environment) after the heating to the fixing belt 103 is stopped due to the shift to the power saving mode. It can be.

In the coil abnormality detection process, the power supplied to the exciting coil 104 is larger than that in the normal mode, and although the temperature of the fixing belt 103 may be higher than that in the normal mode, the temperature of the fixing belt 103 may be higher. If it is controlled to start in a state where the surface temperature is lowered to about room temperature, the temperature rise of the fixing belt 103 can be suppressed as much as possible.
Furthermore, depending on the device configuration, even if the amount of power supplied to the exciting coil 104 is the same as that in the normal mode, it may lead to a discharge due to a rare short, and with this configuration, regardless of the transition to the power saving mode, For example, the coil abnormality detection process can be executed in the normal mode (standby or image forming operation). Of course, in this configuration, control for increasing the amount of power supplied to the exciting coil 104 is not executed. The voltage of the alternating current flowing through the excitation coil 104 is input to the coil short detection unit 85, the high frequency current superimposed on this voltage is detected by the coil short detection unit 85, and the detection result is sent to the CPU 52 of the control unit 50. This is the same as the above embodiment.

In the power saving mode, the power supply to the exciting coil 104 is stopped. However, the present invention is not limited to this. For example, the power supply amount is reduced compared to the normal mode, and the temperature of the fixing belt 103 is lower than the fixing temperature. The mode for executing the control maintained in the above may be the power saving mode.
(2) In the above embodiment, a message is displayed on the liquid crystal display 61 of the operation panel 60 as an output indicating that a trouble (abnormality) has occurred in the exciting coil 104. What is necessary is just to be able to notify, and it is not restricted to the display of a message.

  For example, instead of a message, a configuration may be used in which a lamp indicating that is turned on, or a sound indicating that may be output. It is also possible to adopt a configuration in which information indicating that fact is transmitted by e-mail or the like to an external terminal device used by an administrator or serviceman of the device via a network. Further, two or more of display, voice, and transmission can be combined to produce an output to that effect.

In addition, a circuit is provided that cuts off the power supply to the excitation coil 104, and when the occurrence of a trouble is output to the circuit, the power supply to the excitation coil 104 is forcibly cut off in the circuit. You can also.
(3) In the above-described embodiment, the configuration example in which the fixing belt 103 and the excitation coil 104 are arranged with a constant interval (for example, several mm) is described, but the present invention is not limited to this.

For example, the belt / coil interval is made wider than the normal interval at the time of printing only when a mechanism capable of changing the interval between the fixing belt 103 and the excitation coil 104 (belt / coil interval) is used to detect coil abnormality. It is conceivable to adopt a configuration for performing the control.
If the belt / coil interval is made wider than the normal interval, the excitation coil 104 moves away from the fixing belt 103, whereby the inductance of the excitation coil 104 increases compared to the normal interval, but the resistance component decreases. Therefore, the peak voltage of the current flowing through the exciting coil 104 is higher than that at a regular interval. If the peak voltage increases, the voltage of the high-frequency current superimposed thereon also increases, so that it becomes easier to detect the component of the high-frequency current, and the determination of the occurrence of a rare short can be made with higher accuracy.

  As a mechanism capable of changing the belt / coil spacing, for example, a holding member (not shown) that holds the exciting coil 104 is supported while being movably supported in the perspective direction with respect to the rotation axis of the fixing roller 101. By moving the excitation coil) in the perspective direction by the driving force of an actuator such as a linear motor, the coil / belt interval is switched to a regular interval when printing, and to a wider interval than normal when a coil abnormality is detected. be able to.

  (4) In the above embodiment, an example in which the image forming apparatus according to the present invention is applied to a tandem color digital printer has been described, but the present invention is not limited to this. Regardless of color or monochrome image formation, the electromagnetic induction heating layer 108 provided on the fixing member such as the fixing belt 103 is heated by the magnetic flux generated from the exciting coil 104 to fix the unfixed image on the conveyed sheet S. Any image forming apparatus having an electromagnetic induction heating type fixing unit that fixes the sheet by the heat of the member can be applied to, for example, a copying machine, a FAX, and an MFP (Multiple Function Peripheral). The fixing member is not limited to a belt shape, and may be a roller or a drum shape.

Furthermore, although the litz wire formed by twisting a plurality of strands is used as the exciting coil 104, the present invention is not limited to this, and a configuration in which one strand is wound several times may be used. Moreover, the structure in which the process in said embodiment is performed by software may be sufficient, and the structure which can be performed using a hardware circuit may be sufficient.
In the above embodiment, (a) the CPU 52 of the control unit 50 instructs the switching CPU 84 of the inverter power supply unit 80 to perform switching, and (b) the switching CPU 84 that receives the instruction switches the switch element 83. At the same time, the presence / absence of a rare short circuit is determined based on the signal from the comparator 88, the determination result is notified to the CPU 52, and (c) the CPU 52 outputs the fact that a rare short circuit has occurred based on the determination result. However, the present invention is not limited to this. For example, one CPU may execute the functions. Needless to say, the values of the power value, frequency f, period t, T, time t1, t2, t11, t21, Vpp, and the like are not limited to the above numerical values, and are suitable for each device configuration. Value is determined.

  The contents of the above embodiment and the above modification may be combined.

  The present invention is widely applied to an image forming apparatus in which an electromagnetic induction heat generating layer provided on a fixing member is heated by a magnetic flux from an exciting coil and an unfixed image on a conveyed sheet is fixed on the sheet by heat of the fixing member. can do.

1 Printer 40 Fixing Unit 50 Control Unit 52 CPU
55 Fixing execution time measurement unit 56 Fixing accumulated time storage unit 57 Timer 60 Operation panel 61 Liquid crystal display unit 80, 201, 301 Inverter power supply unit 83 Switch element 84, 302 Switching CPU
85 Coil Short Detection Unit 101 Fixing Roller 102 Pressing Roller 103 Fixing Belt 104 Excitation Coil 105 Fixing Motor 108 Electromagnetic Induction Heating Layer 115, 251 Winding 203 Switch 250 Demagnetizing Coil 303 High Withstand Voltage Relay G Voltage Waveform at Switching Element Off Time L Maximum sheet width L1 Passing area when passing small size paper L2 Non-passing area when passing small size paper S Sheet t, T Switching cycle t1, t11, t12 Switch element on time t2 Switch element off time

Claims (14)

An image forming apparatus that heats an electromagnetic induction heating layer provided on a fixing member by a magnetic flux emitted from an exciting coil, and fixes an unfixed image on a conveyed sheet to the sheet by heat of the fixing member,
Detecting means for detecting an alternating current having a frequency corresponding to a high-frequency current associated with a discharge during a short circuit between windings in the exciting coil;
When the current is detected by the detection means, output means for outputting a trouble in the excitation coil;
An image forming apparatus comprising: The detection means includes
The detection of the current is performed by supplying power to the excitation coil within a predetermined period other than during the fixing operation of the unfixed image on the sheet. Image forming apparatus. During the fixing operation, power of a first magnitude is supplied to the excitation coil to maintain the temperature of the fixing member at a fixing temperature necessary for the fixing operation,
The detection means includes
3. The image forming apparatus according to claim 2, wherein the current is detected by supplying electric power having a second magnitude larger than the first magnitude to the exciting coil. In a period other than during the fixing operation, the power supplied to the exciting coil is suppressed to a third power smaller than the first magnitude, and the temperature of the fixing member is reduced to be lower than the fixing temperature. It is possible to execute a power saving mode that lowers the temperature,
During a predetermined period other than during the fixing operation,
A period during execution of the power saving mode;
The detection means includes
When detecting the current during the execution of the power saving mode, the power supplied to the exciting coil is switched from the third magnitude to the second magnitude for a predetermined time, and then The image forming apparatus according to claim 3, wherein the image forming apparatus is returned to a size of 3. The detection means includes
5. The image forming apparatus according to claim 3, wherein a current having a frequency corresponding to the high-frequency current is detected by detecting a current flowing through the excitation coil. 6. A rectifier that rectifies an AC voltage from an external commercial power source and supplies the rectified current to the exciting coil;
A resonance capacitor connected in parallel to the excitation coil and forming a resonance circuit with the excitation coil;
A switching element connected in series to the excitation coil and opened and closed at a predetermined cycle higher than the frequency of the AC voltage,
The power supply unit is
When the switch element is closed, a current flows through the series circuit composed of the excitation coil and the switch element, and the current flowing through the switch element is cut off by switching from the closed state to the open state. A circuit configuration in which current flows in the circuit,
The detection means includes
The image forming apparatus according to claim 5, wherein a current flowing through the resonance circuit is detected as detection of a current flowing through the excitation coil. The voltage waveform of the current flowing through the resonance circuit in the off time when the switch element is open rises from the start of the off time, turns down after a peak, and reaches the end of the off time. And
The time point when the start of the rise is α1, the time point when the end of the fall is α2, the time point when the predetermined time β1 has elapsed from the time point α1 is γ1, and the time point that is a predetermined time β2 from the time point α2 is γ2. When
The detection means includes
In the voltage waveform in the OFF time of the switch element, a current having a frequency corresponding to the high-frequency current is detected based on the detected voltage value of the waveform portion between the time point γ1 and the time point γ2, and the time point α1 to the time point γ1. The current of the frequency corresponding to the high-frequency current is not detected based on the detected value of the voltage of the waveform portion between the time point γ2 and the time point α2 between the time point γ2 and the time point α2. Image forming apparatus.   The image forming apparatus according to claim 7, wherein the predetermined times β1 and β2 are times corresponding to 10% of the off time. The voltage waveform of the current flowing through the resonance circuit in the off time when the switch element is open rises from the start of the off time, turns down after a peak, and reaches the end of the off time. And
The detection means includes
The voltage waveform in the OFF time of the switch element is characterized in that a current having a frequency corresponding to the high-frequency current is detected based on a detected value of a voltage of a waveform portion that is the peak or a voltage equal to or higher than a predetermined value. 6. The image forming apparatus according to 6. A rectifier that rectifies an AC voltage from an external commercial power source and supplies the rectified current to the exciting coil;
A resonance capacitor connected in parallel to the excitation coil and forming a resonance circuit with the excitation coil;
A switch element connected in series to the excitation coil;
A high-breakdown-voltage switch element that is interposed between the excitation coil and the switch element in the series circuit of the excitation coil and the switch element and has a higher withstand voltage than the switch element;
A power supply unit having
The power supply unit is
A series circuit including the switch element from the exciting coil through the high withstand voltage switch element by switching the switch element from the open state to the closed state from the open state and the high withstand voltage switch element in the closed state. Current flows through
The high withstand voltage switch element is switched from closed to open when a predetermined time elapses from the switching, whereby the current flowing through the high withstand voltage switch element is interrupted, and the current flows through the resonance circuit including the exciting coil,
The detection means includes
The image forming apparatus according to claim 5, wherein a current flowing through the resonance circuit is detected as detection of a current flowing through the excitation coil. The voltage waveform of the current flowing through the resonance circuit after the time when the high-breakdown-voltage switching element is switched from closed to open is a mountain-shaped waveform that rises from the switching time and then goes down through a peak.
The detection means includes
11. The image formation according to claim 10, wherein in the voltage waveform, a current having a frequency corresponding to the high-frequency current is detected based on a detected value of a voltage of a waveform portion that is the peak or a voltage equal to or higher than a predetermined value. apparatus. A demagnetizing coil for generating a magnetic flux for canceling the magnetic flux from the exciting coil;
The excitation coil extends along the sheet width direction orthogonal to the sheet conveying direction,
The degaussing coil is on the end side in the extending direction of the exciting coil, and in the extending direction, a non-sheet passing region outside the sheet passing region of a sheet having a predetermined size or less among the sheets used. It is erected on the corresponding coil part,
The detection means includes
5. The image forming apparatus according to claim 3, wherein a current having a frequency corresponding to a high-frequency current accompanying the discharge is detected by detecting a current flowing through the degaussing coil. A rectifier that rectifies an AC voltage from an external commercial power source and supplies the rectified current to the exciting coil;
A resonance capacitor connected in parallel to the excitation coil and forming a resonance circuit with the excitation coil;
A switching element connected in series to the excitation coil and opened and closed at a predetermined cycle higher than the frequency of the AC voltage,
The power supply unit is
When the switch element is closed, a current flows through the series circuit composed of the excitation coil and the switch element, and the current flowing through the switch element is cut off by switching from the closed state to the open state. A circuit configuration in which current flows in the circuit,
The detection means includes
The image forming apparatus according to claim 12, wherein a current flowing through the degaussing coil is detected by electromagnetic induction when a current flows through the resonance circuit. The detection means includes
The detection of the current is performed only when the value indicating the cumulative value of the power feeding time to the exciting coil reaches a predetermined value within the predetermined period. The image forming apparatus according to claim 1.

Images